The present invention relates to recovery furnaces and particularly to apparatus for automatically cleaning spouts that drain molten chemical-containing smelt from the firebox of a chemical recovery furnace.
Wood pulp for papermaking is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor, " are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor," containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the papermaking process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced dries the spent cooking chemicals as they fall to the floor of the firebox, where they build a mound of material called a char bed. The char bed is further heated to liquify the chemicals into a molten smelt that flows out through a smelt spout in the bottom of the furnace to a collection tank. Concurrently, combustion heat of the furnace is employed to generate steam in an interiorly disposed water-wall tubs boiler for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80 percent of the material entering the furnace. The air is forced into the fire box from wind boxes or ducts disposed at several levels in surrounding relationship to the firebox, through a plurality of air ports in the walls of the furnace, viz.: primary, secondary and tertiary air ports. The primary air ports, through which about 40 to 50 percent of the air enters the furnace, are disposed on the side walls of the firebox near the bottom of the furnace close to the char bed. The secondary air ports, through which about 35 percent of the air enters the furnace, are disposed around the walls of the firebox, higher than the primary air ports, and below the entry conduits through which the black liquor is sprayed into the furnace. While the primary air ports provide a relatively large volume of air with considerable turbulence for maintaining a fireball in the char bed, the secondary air ports provide a finer control and distribution of air above the char bed and distribute the air evenly in the black liquor spray to support the combustion thereof.
The black liquor sprayed into the firebox, having a consistency like warm 60 weight oil, swirls, burns and falls toward the bottom of the firebox in the form of combustion products comprising char material and smelt. Some smelt and char material contact the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around edges of the air ports. In accordance with customary practice, the char buildup is periodically dislodged, either manually by a worker inserting a rod into the air ports around the boiler, or by automatic cleaning apparatus. The dislodged char material falls into the smelt pool in the bottom of the furnace. Smelt spouts are designed to drain the smelt from the furnace, and keep the smelt pool at a safe level. If a smelt spout is not cleaned periodically, the smelt oxidizes and forms a crust which will, in time, clog the smelt spout. Clogged smelt spouts can cause the level of smelt inside the furnace to rise, resulting in inefficient and unpredictable furnace operation, with an attendant decrease in the amount of chemicals that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as carbon monoxide and sulfur dioxide.
Smelt spouts of chemical-recovery furnaces have heretofore been cleaned manually by a worker inserting a long metal rod into the spout. Vigorous lateral and reciprocating movement of the rod by the worker dislodges any char or encrusted smelt that may be clogging the spout. Such manual rodding of the smelt spout is inefficient and also unsafe. The temperature of the smelt is 800 to 1000 degrees Celsius, and although the rods utilized to clean the spouts are in excess of 6 meters long, there is danger that a worker who manually rods the smelt spout can be burned. Smelt spout openings are cooled by water circulating in a jacket surrounding the spout, and the water jacket can be ruptured by the manual rodding operation. A broken water jacket may result in an explosion of the recovery furnace because of the volatility of the smelt in the presence of water.
Apparatus for cleaning openings in a recovery furnace are known--see my U.S. Pat. No. 4,423,533, entitled FURNACE AIR PORT CLEANER; however, such known apparatus are not suitable for cleaning smelt spouts. Conventional punching devices, rods or cleaners inserted into the smelt spouts are inefficient and unsafe to use.
It is accordingly an object of the present invention to provide improved apparatus for cleaning smelt spouts of a chemical recovery furnace.
It is another object of the present invention to provide improved smelt-spout cleaning apparatus installable for automatic operation in a smelt spout of a chemical recovery furnace.
Another object of the present invention is to provide improved smelt spout cleaning apparatus for increasing the operational stability of a black liquor recovery boiler.
It is a further object of the present invention to provide improved automatic smelt spout cleaning apparatus that will reduce the exposure of workers to hazardous areas of the chemical recovery furnace.
Yet another object of the instant invention is to provide improved automatic smelt spout cleaning apparatus that is safe to operate, and virtually eliminates the danger of rupturing a smelt spout cooling-water jacket.
Another object of the present invention is to provide improved automatic smelt spout cleaning apparatus that is relatively light in weight and can be manually installed on or removed from a chemical recovery furnace quickly and easily.